New types of IC (internal combustion) engines, both diesel and spark ignition, are being designed to burn alternative fuel mixtures. including pure hydrogen, and will present new, more challenging operating environment: (increased temperatures and corrosive gas content) for exhaust valves. Of the many components of IC engines, the engine exhaust valve is and will be one of the most challenging material systems. Each exhaust valve must resist exposure to hot (1400-1600 degrees F.) oxidizing combustion exhaust and must achieve and retain a challenging set of physical properties, including resistance to high cycle fatigue, extreme surface wear. and long-term creep deformation.
Current exhaust valves are multi-component material systems that consist of different Fe-based alloys that are joined and coated with several types of oxidation and wear resistant layers and their manufacture has been optimized for current vehicle operating environments.
Some advanced materials been proposed over the years as substitutions for existing materials to extend the lifetime or improve the performance of exhaust valves, including cast/wrought Ti alloys and oxide dispersion hardened Fe-base superalloys that are consolidated from mechanically alloyed powder (metal/oxide) blends.
Dispersoid strengthened metallic material typically comprises a metal or alloy matrix having dispersoids distributed uniformly throughout for strength enhancing purposes. The mechanical alloying (MA) process, particularly at full industrial scale, to make disperoid strengthened materials can add considerable cost to the process of making some very attractive alloys for high temperature service in harsh environments. In fact, the largest facilities in the US for making these types of alloys, in Huntington, W. Va., owned by Huntington Alloys, Inc., were recently shut down and put up for sale. Probably the most successful product of the mechanical alloying process is termed MA 956 by Inco Alloys International, Inc., and consists of Fe-20Cr-4.5Al-0.5Ti-0.5Y2O3-0.05C (in wt. %) that is an Fe-based alloy with dispersed Y2O3 particles for retained high temperature strength. In the manufacture of mill forms of MA 956 for example, the starting MA particulate is produced from a blend of Fe, Cr, and master alloy (Al—Ti, and Fe—C) powders, along with the addition of Y2O3 powder, which is milled for extended times (days) in a high energy milling unit, e.g., horizontal ball mill. Typically, the highly refined composite powders that result are consolidated by direct hot extrusion. Subsequent thermo-mechanical treatment, e.g., by hot rolling and high temperature heat treatment (1300 degrees C.) is needed to promote secondary recrystallization of the microstructure. This is needed to grow the grain size far coarser than the interparticle spacing of the dispersed Y2O3 particles (25 nm), which imparts some useful ductility to the final machined parts. Both the milling equipment and extensive milling time are very costly. well beyond normal ingot metallurgy processing steps for this class of alloys (either stainless steels or Ni-base superalloys) without dispersoids, although their high temperature strength retention can be superior.
Powder metallurgy methods represent one of the most cost effective materials processing approaches for mass production of high performance engine components, e.g., the universal displacement of cast steel by powder metallurgy (pressed/sintered/forged) processed steel for IC engine connecting rods.
Gas atomization is a commonly used technique for economically making fine metallic powder by melting the metallic material and then impinging a gas stream on the melt to atomize it into fine molten droplets that are rapidly solidified to form the powder. One particular gas atomization process is described in the Ayers and Anderson U.S. Pat. No. 4,619,845 wherein a molten stream is atomized by a supersonic carrier gas to yield fine metallic powder (e.g., powder sizes of 10 microns or less). Anderson U.S. Pat. Nos. 5,073,409 and 5,125,574 describe high pressure gas atomization of a melt in a manner to form a thin protective refractory nitride surface layer or film on the atomized powder particles. The '409 patent uses an atomizing gas, such as nitrogen, that selectively reacts with an alloy constituent to form the protective surface layer. The '574 patent uses an inert atomizing gas and a reactive gas contacted with the atomized droplets at a selected location downstream of the atomizing nozzle to form the protective layer. Various prior art techniques for forming protective layers on atomized powder by reacting a gaseous species with the melt, or a component of the melt, are discussed in these patents.
U.S. Pat. No. 5,368,657 discloses a powder making process called gas atomization reaction synthesis (GARS) wherein a superheated melt comprising a metallic material is formed and atomized with an atomizing gas to produce atomized particulates. The atomizing gas can comprise a carrier gas and a reactive gas or liquid that is reactive when dissolved in solid solution in the metallic material to form dispersoids therein. The temperature of the melt and the ratio of the carrier gas to the reactive gas are selected effective to provide a superequilibrium concentration of reactive species, such as nitrogen, in solid solution in at least a surface region of the atomized particulates. The atomized particulates can be heated to a temperature to react the dissolved species with the metallic material to form dispersoids therein. Alternately, the atomized particulates having a superequilibrium concentration of the dissolved species are formed into an article, and the article then is heated to a temperature to react the dissolved species with the metallic material to form dispersoids in the article.
Copending patent application Ser. No. 11/429,918 filed May 8, 2006, describes a method of making dispersoid-strengthened alloy particles in response to both present and anticipated needs for new material systems for IC engine exhaust valves and high temperature structural applications by development of cost effective processing methods for making dispersion strengthened alloy powder particles and products made therefrom having enhanced fatigue and creep resistance and reduced wear for automotive and heavy-duty vehicle applications as well as enhanced corrosion/oxidation resistance at high temperatures. The method involves providing an alloy comprising an environmental (e.g. corrosion or oxidation) resistance-imparting alloying element (e.g. Cr), a dispersoid-forming element (e.g. Y), and a matrix metal (e.g. Fe), wherein the dispersoid-forming element exhibits a greater tendency to react with a reactive species acquired from an atomizing gas than does the alloying element. The alloy is melted and atomized with the atomizing gas containing the reactive species so that the reactive species is reacted with the alloying element to form a surface compound and/or dissolved into solid solution to at least a depth below the surface of atomized particles. Thus secured, the reactive species is available for reaction with the dispersoid-forming element by subsequent particle heating and/or reacted with the dispersoid-forming element in-situ during atomization to form dispersoids in the atomized particles to at least a depth below the surface of the atomized particles. For an alloy comprising Fe-12.5% Cr-1% Y atomized using an atomizing gas mixture comprising argon-5 volume % oxygen, the atomized particles have been found to include about 20,000 ppm by weight oxygen as a chromium oxide surface compound and as Y—Cr—O dispersoids. Once consolidated, lengthy annealing times are required to convert a significant fraction of the chromium oxide to yttrium oxide dispersoids by an exchange reaction. Even with long anneal times, the chromium oxide may not be substantially converted, leaving unwanted chromium oxide in the particles which remain in a consolidated body made from the particles.
The present invention seeks to improve control of the amount of the reactive species, such as oxygen, introduced into the atomized particles so as to reduce anneal times and improve reaction (conversion) to the desired strengthening dispersoids in the matrix, resulting in an ideal microstructure, without significant remnants of the initial alloy surface compound.